In an optical communication field, a multi channel communication is rapidly promoted by the advent of a WDM (wavelength division multiplexing) communication system. In accompaniment by this, optical elements of a number corresponding to the number of channels are needed in order to achieve a functional control of each channel, for instance, controlling power of each channel to a constant power and controlling of switching.
For this reason, small-sized optical circuit parts are highly needed that can be applied to optical switches and enables a high-density integration. A light switch of a single unit has already been practiced. A matrix switch has been practiced in which the large number of these light switches is used and which has a plurality input-output ports. Various techniques are proposed to achieve the light switch as follows: that is, a method of connecting an input port and an output port by mechanically moving them (for instance, Japanese Laid Open Patent Application (JP-A-Heisei 9-5653)), a method of connecting an input port and an output port by rotating a movable mirror to have a predetermined angle (for instance, Japanese Laid Open Patent Application (JP-P2001-255474A), and Electronic Information Communication Academy convention proceedings C-3-8 (2002) p. 140), a method of using liquid crystal (For instance, Japanese Laid Open Patent Application (JP-A-Showa 62-187826)), a method of changing the connections between an input port and an output port by controlling reflection of light by means of generating bubbles in intersection of waveguides, and the like.
Among these techniques, a plan light wave circuit (PLC) type device using a thermo-optic phase shifter is extremely excellent in easiness of manufacturing and integration, and has a feature of advantage for high level functioning and high-density integration because manufacturing technique of a semiconductor circuit can be used for its manufacturing process.
Generally, the thermo-optic phase shifter is manufactured in the following way. First, an optical waveguide consisting of a clad layer and a core layer is formed on a substrate. A metal film and the like are formed on this optical waveguide, and are processed to have a fine line shape along the optical waveguide. When electric power is externally supplied to this thin film, heat is generated by the electric resistance of the thin film to operate as a heater of the optical waveguide. The heat generated by the heater reaches the core layer through the clad layer of the optical waveguide. As a result, refractive index in a portion heated by the heater in the optical waveguide increases, and the effect waveguide length becomes long based on a change amount of the refractive index and a waveguide length, so that the phase of light in an output end is shifted. An amount of phase shift can be arbitrarily controlled by adjusting the electric power supplied to the heater. It should be noted that when the optical waveguide is formed of quartz glass, the refractive index temperature coefficient (dn/dT) of the quartz glass is about 1×10−5 (/° C.).
One optical waveguide is separated into two optical waveguides in an input port, at least one of the two optical waveguides is connected to the thermo-optic phase shifter, and the two optical waveguides are combined in an output end again. As a result, a light switch is practiced. For instance, if the phases of light components propagated in the two optical waveguides are shifted mutually by a half wave length, the output in the output end is zero. Also, in case of no phase shift, the light is outputted as it is inputted. Thus, On/Off control of the output can be achieved.
However, when a plurality of thermo-optic phase shifters are provided in one optical circuit for multiple channels, the electric power consumption of the entire optical circuit increases extremely if each thermo-optic phase shifter consumes large electric power. For instance, when light with the wave length of 1550 nm is guided which is usually used for optical communication, the electric power of about 400 mW per a channel is necessary to shift the phase of light by a half wave length in the conventional thermo-optic phase shifter. Therefore, for instance, the electric power of 40*400 mW=16000 mW=16 W is needed in maximum to control the optical communication circuit with 40 channels, when a switch using the above-mentioned thermo-optic phase shifter is provided in every channel. The thermo-optic phase shifter having the electric power consumption of about 40 mW per a channel is reported in research stage. However, the electric power consumption is still too large for the requirement of high integration to the thermo-optic parts.
To reduce the electric power consumption of the thermo-optic phase shifter, a method of changing material for forming the optical waveguide into a material with large temperature coefficient of refractive index is proposed. For instance, a method of using polymer for the waveguide is proposed (for example, Japanese Patent No. 2,848,144, IEEE Photon. Technol. Lett. by Y. Hida et al. (Vol. 5 (1993) pp. 782-784), and The Electronic Information Communication Academy convention proceedings C-3-10 (2002) p. 142).
Moreover, the thermo-optic phase shifter is also proposed in which a groove is provided between the optical waveguides to prevent heat generated by the heater from being transferred externally (for instance, The Electronic Information Communication Academy convention proceedings C-3-61 (2001) p. 226, the Electronic Information Communication Academy convention proceedings C-3-64 (2001), p. 229, IEEE Photon. Technol. Lett. by Q. Lai et al. (Vol. 10 (1998) pp. 681-683)). According to these references, a desirable temperature increment quantity can be obtained with smaller electric powers by providing a groove.
In addition, a method of thickening a clad layer located under a core layer is also proposed to prevent heat generated by a heater from being transferred to a substrate. Moreover, a technique is disclosed in which the surface of a substrate under an optical waveguide is removed to have a bridge structure in the thermo-optic phase shifter formed on a silicon substrate, in order to prevent the heat originated by the heater from being transferred to the substrate (for instance, Japanese Laid Open Patent Application (JP-A-Heisei 1-158413), Japanese Laid Open Patent Application (JP-A-Heisei 5-34525), and Japanese Laid Open Patent Application (JP-P2001-222034A)). Furthermore, a technique is disclosed in which a part of a silicon substrate under an optical waveguide is left to form a pole for supporting the optical waveguide to the silicon substrate in the paper by A. Sugita et al. (Trans. IEICE, Vol. E73 (1990) pp. 105-109).
In addition, Japanese Patent No. 3,152,182 discloses the following technique. That is, a silicon thin film is selectively formed on a quartz substrate, and an under clad layer is formed to cover this silicon thin film. A core is formed above the silicon thin film on the under clad layer, and an over clad layer is formed to cover the core layer. Thus, the optical waveguide is formed, and a heater is formed on the optical waveguide. Then, grooves are formed to reach the silicon thin film so as to put the optical waveguide between the grooves and the silicon thin film is removed by using the grooves. Thus, a gap is formed between the optical waveguide and the quartz substrate to reduce the electric power consumption of the thermo-optic phase shifter.
However, the above-mentioned conventional techniques have the following problems. That is, when the optical waveguide is formed of polymer, film quality of the polymer is deteriorated because of the high hygroscopic property of the polymer so that the polymer absorbs the moisture during manufacturing and operation of the thermo-optic phase shifter. For this reason, the polymer optical waveguide has larger propagation loss of light, compared with the optical waveguide formed of quartz glass. Also, it is difficult to form a passivation protection film on the polymer optical waveguide. Therefore, the polymer optical waveguide has lower stability and inferiority in reliability, compared with the quartz glass optical waveguides. Moreover, a method of burying polymer partially in the quartz glass optical waveguide may be proposed. However, this method causes various problems such as complication of a manufacturing process, low reproducibility, and increase of propagation loss occurred in an interface of quartz glass and polymer.
Moreover, in the method of providing the grooves between the optical waveguide, it is possible to prevent that the heat from the heater provided directly on a certain optical waveguide conducts another adjacent optical waveguide. However, it is not possible to prevent heat from the heater from being transferred to the substrate. Thus, the effect to decrease the electric power consumption becomes small.
In addition, in the method of thickening the clad layer under the core layer, there is a problem that a crack generates with a stress generated in the clad layer during the film growth. Moreover, there is a problem of causing a bend of the substrate with the stress. Furthermore, an optical characteristic of the optical waveguide is deteriorated because of this stress. In addition, it is not suitable for mass production because the deposition time becomes long. Therefore, it is difficult to form the clad layer thick in the process.
In addition, in the technique of removing the surface of the silicon substrate under the optical waveguide, the strong acid such as fluorinated nitric acid is needed as etchant in order to etch the silicon substrate. The heater is protected by being covered with a resist layer in the etching of the silicon substrate. The resist layer cannot endure the fluorinated nitric acids and the heater receives damage through the etching. Thus, there is a problem in the process in the method of the etching of the silicon substrate. Moreover, the partial removal of the silicon substrate leads to weaken strength of the substrate itself for securing strength of the thermo-optic phase shifter. As a result, mechanical strength of elements is reduced. Also, the etching of the silicon substrate leads to an unstable state of the optical waveguide since the state of impression of the stress to the clad layer changes. As a result, mechanical strength and an optical characteristic of the clad layer of the optical waveguide are deteriorated. In addition, if a part of the silicon substrate is remained as a pole as described in the paper by A. Sugita et al. (Trans. IEICE Vol. E73 (1990) pp. 105-109), a thermal insulation effect to the optical waveguide is acutely deteriorated because silicon has high thermal conductivity. Thus, the original purpose of reducing power consumption cannot be accomplished.
Furthermore, in the technique disclosed in Japanese Patent No. 3,152,182, i.e., in which a silicon thin film is provided on the substrate selectively, and a gap is formed between the substrate and the optical waveguide by etching the silicon thin film in a subsequent process, it is a problem that the etching of the silicon thin film is still difficult. Moreover, the upper surface of an under clad layer does not become flat because the under clad layer is formed to cover the silicon thin film. Thus, there is a problem that it is difficult to form the core layer, an over clad layer, and a heater on the under clad layer.
In conjunction with the above description, a light switch and a manufacturing method of the same are disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-148536). The conventional light switch has a movable member that is electrically conductive and elastically deformable is supported in the shape of cantilever. An electrode is provided adjacent to the movable material and drives between a first position and a second position with electrostatic force. A stop member stops the movable material at each position of the first position and the second position. A first optical waveguide is formed in the movable material. A second optical waveguide is optically coupled to the first optical waveguide when the movable material is in the first position. A third optical waveguide is optically coupled to the first optical waveguide when the movable material is in the second position.
Also, a semiconductor mechanics sensor and a manufacturing method of the same are disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-201984). The conventional semiconductor mechanics sensor includes a support substrate, a beam structure, a movable electrode and a fixed electrode. The beam structure is supported on the support substrate in an electrically insulated state from the support substrate and is formed of a semiconductor material to displace in accordance with a mechanics amount. The movable electrode is provided as a unitary body with the beam structure. The fixed electrode is supported on the support substrate in an electrically insulated state from the support substrate and is formed of the semiconductor material. The mechanical force that acts on the beam structure is detected based on the change in electrostatic capacitance between the movable electrode and the fixed electrode in accompaniment by the displacement of the beam structure. A signal output section is provided on the support substrate in an electrically insulated state from the support substrate. The signal output section and the fixed electrode are connected by a wiring film with an air bridge structure that is formed of polycrystalline semiconductor material.
Moreover, a manufacturing method of an optical waveguide with a phase adjustment function is disclosed in Japanese Patent No. 3,204,493. The conventional optical waveguide with the phase adjustment function includes a clad layer formed on a substrate, a core layer buried in the clad layer, and a phase adjustment heater provided above the core layer to adjust optical path length. In the method of manufacturing the optical waveguide with the phase adjustment function, a part of the substrate is removed by a mechanical process and a removal region is formed. A substrate with a thermal conductivity lower than the substrate is formed by a mechanical process to suit the removal region. Then, both of the substrates are combined as a unitary body and a composite substrate is formed. The optical waveguide that consists of the clad layer and the core layer is formed on the composite substrate. The phase adjustment heater is formed above the substrate with low thermal conductivity.